Universal N-glycan binding reagent

ABSTRACT

Methods of capturing N-glycan linked glycomolecules including N-glycans, N-glycopeptides and N-glycoproteins are described. The methods provide substantially unbiased capture of charged and uncharged N-glycans and/or N-glycan linked glycomoleules. Binding reagents for substantially unbiased binding of N-glycans and/or N-glycan linked glycomolecules are also described.

CROSS REFERENCE

This application is a continuation of U.S. application Ser. No.15/320,911, filed Dec. 21, 2016, which is a § 371 application ofInternational Application No. PCT/US2015/37960 filed Jun. 26, 2015,which claims priority from U.S. Provisional Application No. 62/020,335,filed Jul. 2, 2014, herein incorporated by reference.

BACKGROUND

Glycosylation is one of the most common and structurally diverse formsof protein post-translational modification. Glycans can be N-linked toan asparagine residue, or O-linked to either a serine or a threonineresidue. Glycans on any given protein can vary widely in structure,composition and the site of attachment on a protein. Individual siteswithin one protein type may even contain a heterogeneous population ofglycan modifications. This complexity interferes with readily analyzingand characterizing glycosylated proteins. Separating glycans from aglycosylated protein is a common first step in characterizing theglycans present on a glycoprotein. However, this method yields noinformation about the glycan attachment site on the protein.

Chemical methods exist for capturing glycans, glycopeptides andglycoproteins. For example, hydrazide chemistry (see for example Zhang,et al., Nat. Biotechnol., 21, 660-666 (2003)) involves periodateoxidation of carbohydrate cis-diol groups to dialdehydes and covalentcoupling between aldehyde and hydrazide groups to form hydrazine bondson a solid support. Alternatively, mild periodate treatment can oxidizesialic acids on glycans allowing capture of the oxidized glycoproteinsby covalent bonding with hydrazide (see for example, Nilsson, et al.,Nature Methods, 6, 809-813 (2009)). However, this method is selectivefor only glycans with sialic acids and the glycans are also chemicallyaltered by the oxidation process. Ideally, glycomolecule composition isanalyzed without chemical modification. Boronic acid reacts withcis-diol-containing saccharides or polyols to form five- or six-memberedcyclic esters and this property has been exploited to isolateglycoproteins and glycopeptides. Importantly, the covalent linkage iseasily reversible in an acidic pH (Chen, et al., Analyst, 139, 688(2014)). However, these aforementioned chemical methods unfortunately donot discriminate between N-linked and O-linked glycomolecules.

In titanium dioxide chromatography enrichment, negatively charged sialicacid residues coordinate with the titanium metal ion (for exampleLarsen, et al., Mol. Cell. Proteomics, 6, 1778-1787 (2007)). However,metal ion affinity chromatography (using titanium, zirconium or silver)is not selective for glycans since negatively charged phosphopeptides orpeptides with acidic amino acids, such as glutamic acid and asparticacid may compete for binding.

HILIC enrichment is a common approach for enriching glycans whereby awater-miscible organic solvent (typically acetonitrile) achievesseparation of glycans via a partitioning mechanism. Hydrophilic sugarresidues partition into the aqueous phase and are attracted to thehydrophilic groups on a solid support (typically silica or derivatizedsilica). HILIC materials show broad specificity for glycans but do notdiscriminate between O-glycan linked and N-glycan linked glycomolecules(Chen, et al., Analyst, 139, 688 (2014)).

Finally, lectins are proteins with natural carbohydrate bindingproperties. Many lectins have been characterized and several have beenemployed for glycopeptide/glycoprotein enrichment or detection. Lectinsrecognize the variable region of N-linked glycans and the specificity ofa lectin may be quite narrow (L-phytohemagglutinin (L-PHA) for thetargeted beta-1,6-branched N-linked glycan (see for example, Ahn, etal., Anal. Chem., 82, 4441-4447 (2010)) or relatively broad in the caseof Concanavalin A, which recognizes a high mannose structure.Nevertheless, a diverse set of lectins with selective affinities forspecific carbohydrate epitopes has been used to investigate the humanglycoproteome. However, to this date no single lectin has been shown topossess sufficient selectivity to analyze the entire N-glycan linkedglycoproteome. Another major drawback of existing lectin basedenrichment methods is low affinity of most natural lectins for theirsubstrates (Kd ranging from 10 mM to 1 μM, (see for example, Fanayan, etal., Electrophoresis, 33, 1746-1754 (2012)). Elution of boundglycomolecules from lectins may be achieved by low pH, for exampleglycine-HCl buffer (at pH 2-2.8) or 100 mM acetic acid; yet low pHexposure can potentially alter glycan structure. Alternatively,glycomolecule elution from lectins can be accomplished using theappropriate sugar to displace the bound glycomolecule from theimmobilized lectin but the added sugar will complicate most downstreamanalyses.

Typically mass spectrometry analysis of a sample having a mixture ofpeptides and glycopeptides (both O-linked and N-linked) reveals a highlycomplex pattern of peaks. The primary problem is that this complexpattern cannot be interpreted to identify and characterize theindividual glycomolecules in the sample. Therefore, a need exists for anenrichment reagent that is able to selectively isolate an individualclass of glycomolecule, for example either N-glycan linkedglycomolecules or O-glycan linked glycomolecules. Upon fractionation ofa complex sample, the results of the mass spectrometry analysis might bemore easily interpreted.

Many important biological activities are affected by proteinglycosylation, including protein folding, protein metabolism,protein-protein interactions, immune cell recognition and intercellularsignaling. Given the emerging interest in glycoproteins as biomarkers, aneed exists for readily analyzing and characterizing proteinglycosylation.

SUMMARY

In general, a fusion protein is provided having an amino acid sequencewith at least 90% identity to SEQ ID NO: 1 and linked to animmobilization module. A protein having an amino sequence with at least90% sequence identity with SEQ ID NO: 1 is exemplified by a proteinhaving at least at least 90% identical to SEQ ID NO: 2.

In a further example of the fusion protein described above, theimmobilization module may be a variant of O⁶-alkylguanine-DNAalkyltransferase (AGT). Also see SEQ ID NOs: 3 and 4.

In general a protein is provided that is a sequence variant of SEQ IDNO: 2, having one or more mutations, wherein a mutation is positioned atone or more positions selected from position 154, 155, 156, 173, and 174for example wherein the mutation at position 154 is a P, at position 155is an A, G or P at position 156 is an S or an R, at 173 is a Y or atposition 174 is an R. In one aspect, the protein is fused to animmobilization module. In one aspect, the immobilization module is avariant of AGT.

In general, a method is provided that includes the steps of a) combininga binding reagent capable of selectively binding charged and/oruncharged N-glycans or N-glycan linked glycomolecules and not bindingany O-glycans; b) binding with substantially no bias, the charged oruncharged N-glycan glycomolecule to the binding reagent in a firstbuffer; and c) releasing the N-glycan glycomolecules from the bindingreagent with a second buffer which does not comprise SDS or anoligosaccharide. The first and second buffers can be volatile for massspec. High salt conditions may be preferred depending on the bindingreagent. The methods and reagents need not chemically alter theN-glycans before capture. The captured N-glycans and/or N-glycan linkedglycomolecules can be readily analyzed for glycan structure and/orglycan attachment site information.

In one aspect a method is provided that includes: a) combining a complexmixture comprising N-glycans and/or N-glycan linked glycomolecules witha binding reagent that selectively binds a core pentasaccharide,Man(α1-3)(Man(α1-6))Man(β1-4)GlcNAc(β1-4)GlcNAc (abbreviated asMan3GlcNAc2 or M3N2), in N-glycans; b) capturing the corepentasaccharide in N-glycans and/or N-glycan linked glycomolecules bythe binding reagent in a first buffer for substantially unbiased bindingof charged and uncharged N-glycans in the glycomolecules; and c)releasing the bound N-glycans and/or N-glycan linked glycomolecules fromthe binding reagent with a second buffer which does not comprise SDS oran oligosaccharide.

In one aspect of the method, the binding reagent comprises an amino acidsequence with at least 90% identical to SEQ ID NO: 1. In another aspectof the method, the binding reagent comprises an amino acid sequence withat least 90% sequence identity to SEQ ID NO: 2. In another aspect, thebinding reagent comprises an amino acid sequence with at least 90%sequence identity to SEQ ID NO: 2 and a mutation positioned at one ormore positions selected from position 154, 155, 156, 173, and 174. Inanother aspect, the binding reagent further comprises an immobilizationmodule.

In another aspect, the immobilization module is an AGT. In anotheraspect, the binding reagent is immobilized.

In some embodiments, the binding reagent is free in solution. In someembodiments, the binding reagent bound with N-glycans or N-glycan linkedglycopeptides is separated from other glycans or non-N-glycan linkedglycopeptides or peptides or other small molecules using a filter ormembrane having a discriminating pore size. In other aspects the bindingreagent is immobilized on a matrix.

In one aspect, the glycomolecule is a product of enzyme cleavage,wherein the enzyme is selected from the group consisting of aglycosidase, a sialidase, a DNase, an RNase, or a combination thereof.In one aspect, the cleavage enzyme is selected from the group consistingof is trypsin, endoproteinase GluC, endo proteinase AspN, proteinase K,Factor Xa protease, IdeS, IdeE, enterokinase, furin, endonuclease S,neuraminidase, peptide-N-glycosidase Ar (PNGase Ar),peptide-N-glycosidase A (PNGase A), peptide-N-glycosidase F (PNGase F),O-glycosidase, endoglycosidase D (endo D), endoglycosidase F (endo F),endoglycosidase F1 (endo F1), endoglycosidase F2 (endo F2),endoglycosidase F3 (endo F3), endoglycosidase H (endo H),endoglycosidase S (endo S), beta1-3 galactosidase, beta1-4galactosidase, alpha1-3,6 galactosidase, beta-N-acetylglucosaminidase,alpha-N-acetylgalactosamindiase, beta-N-acetylhexosaminidase, alpha1-2,3mannosidase, alpha1-6 mannosidase, neuraminidase, alpha2-3neuraminidase, alpha1-2 fucosidase, DNase, RNase H, or a combinationthereof. In another aspect, the glycomolecule is a polypeptide which maybe a product of protease digestion.

In one aspect of the compositions and/or methods, the N-glycan linkedglycomolecule is an N-glycan, an N-glycan linked glycopeptide, anN-glycan linked glycoprotein, or a combination thereof. In someembodiments, the N-glycan moiety is labeled, unlabeled, or a mixturethereof. In some embodiments, the N-glycan linked glycomolecule isnative or denatured.

In some embodiments, the N-glycan and/or N-glycan linked glycomoleculesmay be in or from a sample of an in vitro cell culture, a bodily fluid,a bodily secretion, a cell, a tissue, an environmental sample, or acombination thereof. Such samples can be from a human, animal, plant,bacteria, soil sample,

In one aspect, the second buffer for eluting immobilized N-glycansand/or N-glycan linked glycomolecules includes one or more reagentsselected from acetonitrile, water, dichloromethane, dichloroethane,pentahydrofuran, ethanol, propanol, isopropanol, methanol, nitromethane,toluene, DMSO, acetic acid, formic acid or a mixture thereof. In someembodiments, the second buffer comprises acetic acid, formic acid orother acidic solutions. In some embodiments, the second buffer isvolatile. In one aspect, the second buffer includes acetonitrile.

In some embodiments, the buffer for unbiased binding of charged anduncharged N-glycans comprises a high salt concentration. For example, insome embodiments, the high salt concentration is at least 500 mM, 750mM, 1 M, 1.5 M, 2 M, 2.5 M, or 3 M. In some embodiments, the salt isselected from ammonium acetate, ammonium chloride, ammonium sulfate,calcium acetate, calcium chloride, magnesium acetate, magnesiumchloride, magnesium sulfate, potassium acetate, potassium chloride,potassium sulfate, sodium acetate, sodium chloride, and sodium sulfateor a mixture thereof. Volatile reagents in the first and second buffersare preferred for mass spectrometry.

In one aspect, the method includes analyzing the composition of theN-glycan and/or N-glycan linked glycomolecules using an analytical testselected from mass spectrometry, chromatography, electrophoresis,nuclear magnetic resonance spectrometry and fluorescence-mediateddetection, or a combination thereof.

In another aspect, the method includes characterizing at least one of:the structure of the N-glycans, the linkage site of the N-glycan on theN-glycan linked glycomolecule, and/or an amount of one or more differentN-glycans, or a combination thereof.

In general a method is provided for treating a patient with a hearingloss, neurofibrillary tangles and/or virus infection; comprisingadministering an effective dose of a truncated ubiquitin ligase that hasenhanced binding to N-glycans on glycomolecules.

In one aspect, the truncated ubiquitin ligase has a sequence of at least90% sequence identity with SEQ ID NO: 2. In another aspect, thetruncated ubiquitin ligase has one or more mutations at positionscorresponding to any of 154, 155, 156, 173, or 174 of SEQ ID NO: 2.

BRIEF DESCRIPTION OF THE DRAWINGS

The figures and drawings are intended to illustrate one or more versionsof the compositions and/or methods described herein. Unless statedotherwise, these are not intended to be limiting for the purpose ofinterpreting the scope of any claims.

FIG. 1 outlines one example embodiment of the methods described herein.Mass spectrometry analysis of a sample having a mixture of peptides,glycopeptides (both O-glycan linked and N-glycan linked) reveals ahighly complex pattern of peaks (see box labeled “before enrichment”).Complex patterns cannot be interpreted to identify and characterize theindividual glycomolecules in the sample. Here, a clear massspectroscopic analysis is provided in (Step 6) that can identify andcharacterize all N-glycan linked glycomolecules present in the sample(see box labeled “after enrichment”). Step 1 is protease digestion of aglycoprotein to generate fragments where some of the fragments haveglycans while others do not. Mass spectrometry at this stage (Step 5)reveals a complex result. However, if the analysis proceeds to (Step 2)which includes the binding of the glycan peptide fragments to Fbs coatedbeads in a suitable buffer and (Step 3) which involves washing and thenelution of the glycopeptides, then mass spectrometry reveals asimplified pattern which can be readily and accurately analyzed.

FIGS. 2A and 2B demonstrate that Fbs binding to glycomolecules isglycan-dependent.

FIG. 2A is an SDS-PAGE gel showing that RNaseB is bound bybead-immobilized SNAP-Fbs only when the N-linked glycan is present.

Lane 1 is a control (CTL) showing the amount of input RNaseB.

Lane 2 is the result of incubating Fbs beads with RNaseB pre-treatedwith PNGaseF to remove N-linked glycans.

Lane 3 is the result of incubating Fbs beads with untreated RNaseB.RNaseB was eluted by boiling in SDS-containing gel loading buffer (lanes2 and 3).

FIG. 2B is an SDS-PAGE gel showing that human IgG is bound bybead-immobilized Fbs only when N-linked glycan is present. Human IgGtypically contains a glycan modification at amino acid position Asn297of the heavy chain. Thus, Lanes 3 and 4 show that only the heavy chainis bound by Fbs beads and only when glycan is present (Lane 4).

Lane 1 is a loading control showing the mobility of light chain versusheavy chain.

Lanes 2, 3 and 4, show a small amount of SNAP-Fbs protein that isreleased from the beads during boiling in SDS-containing gel loadingbuffer to elute bound heavy chain (see asterisk).

FIG. 3 demonstrates that (alpha 1-6) fucosylation of the corepentasaccharide (M3N2) structure does not affect binding of SNAP-Fbs toN-glycans (see Example 5). Two cartoons show the position of (alpha 1-6)fucosylation that is found at the reducing end of GlcNAc in some nativeN-glycan linked glycomolecules. The isothermal titration calorimetrydata demonstrates that the affinity between the binding reagent and bothM3N2 and fucosylated M3N2 (M3N2F) is nearly the same. The affinity ofthe binding reagent for Man3GlcNAc2 (Kd M3N2=0.123±0.036 μM) is verysimilar to the affinity of the binding reagent for Man3GlcNAc2 modifiedwith fucose (Kd M3N2F=0.128±0.037 μM) indicating that N-glycan binds tothe binding reagent tightly and fucosylation of the first GlcNAc residuefrom the reducing end does not impair binding by the binding reagent.

FIG. 4 shows that 2-aminobenzamide (2-AB) fluorophore labeling at thereducing end of the M3N2 completely abolishes binding by SNAP-Fbs beads.The binding and elution profiles of 2-AB labeled M3N2 that was incubatedwith SNAP-Fbs beads were compared with SNAP only control beads.

FIG. 5 demonstrates that wild type Fbs (wtFbs) binds to complex-typeN-glycans containing sialic acids (see Example 6). The testglycomolecule is sialylglycopeptide (SGP, structure shown). Isothermaltitration calorimetry determines that SNAP-wtFbs (in solution) can bindto SGP, calculated Kd was 3.0+/−0.12 μM.

FIGS. 6A-6C show the properties of wtFbs binding to SGP or SGP-TMR inlow salt and high salt binding buffer.

FIG. 6A shows the unexpected finding that binding affinity of wtFbs toSGP-TMR is improved by a higher salt concentration (2M NaCl). High saltconditions are typically used for elution conditions, not bindingconditions. However, binding of wtFbs to Asialo-SGP-TMR (lacking sialicacid) is equally efficient in low salt (50 mM NaCl) and high salt (2MNaCl).

FIG. 6B shows the effect of increasing amounts of salt (0-3000 mMammonium acetate pH 7.5) on binding of SGP-TMR to wtFbs.

FIG. 6C shows that high salt (2M NaCl) significantly improves thebinding affinity of wtFbs to sialylglycopeptide (SGP) as determined byisothermal titration calorimetry. The effect of high salt binding is agreater than 2-fold change in Kd (3.0+/−0.12 divided by 1.43+/−0.04).

FIG. 7A-7C show that Fbs may be employed for enrichment of N-glycanlinked glycomolecules and subsequent identification and relativequantification by LC-MS (liquid chromatography coupled to massspectrometry).

FIG. 7A is a total ion chromatogram (TIC) and enlargement demonstratingwtFbs-mediated binding and enrichment of N-glycan linked glycopeptidesfrom a complex mixture (see Example 10). The complex mixture was atryptic digest of RNaseB spiked with SGP to serve as a complex,sialylated glycopeptide. RNaseB contains several non-glycosylatedpeptides and several different high mannose N-glycan linkedglycopeptides (labeled in the enlargement as: M5N2, M6N2). The dottedline indicates the chromatogram of the input mixture without enrichment.The solid black line indicates the chromatogram of the high salt (HS=2Mammonium acetate pH 7.5) enrichment sample. The solid gray lineindicates the chromatogram of the low salt (LS=50 mM ammonium acetate pH7.5) enrichment sample. The enlarged box focuses on the glycomoleculesthat elute between 20-22 minutes whereas the non-glycosylated peptideselute before 20 minutes. Strikingly, the non-glycosylated peptides werenot enriched. N-glycan linked glycopeptides present in the sample wereselectively bound by the binding reagent.

FIG. 7B is an extracted ion chromatogram which provides a quantificationof the mass spectrometry data from FIG. 7A. wtFbs binding reagentsignificantly enriched for N-glycan linked glycopeptides and theenriched sample is substantially unbiased when using high salt bindingconditions.

FIG. 7C shows that wtFbs is able to enrich N-glycan linkedglycomolecules from a tryptic digest of transferrin. When using highsalt conditions (2 M ammonium acetate pH 7.5), the recovery of varioustypes of complex glycomolecules ranged from 28.2% to 42.7%.

FIG. 8 compares elution buffers (see Example 10). An organic solvent(50% acetonitrile in this example) and mildly acidic conditions (1%acetic acid in this example) were significantly more efficient ateluting bound N-glycans from the binding reagent than high saltconditions (2 M ammonium bicarbonate), which was the least efficient.The test N-glycan linked glycopeptide was SGP-TMR. The y-axis is thepercent of eluted labeled SGP. About 78% of SGP-TMR was eluted in thefirst 100 μl of 50% acetonitrile (ACN=acetonitrile; HAc=acetic acid).

FIGS. 9A-9C serve to describe various Fbs mutants with improved bindingaffinity for N-glycan linked glycomolecules.

FIG. 9A shows the amino acid sequence of Fbs sugar binding domain (SBD).The SBD (position 113 to 296 in human Fbs) was expressed as a fusion toeither p50 DNA binding protein or to the SNAP-tag. Bold letters indicatethe positions that were subject to mutagenesis. The table shows themutants of human Fbs that were analyzed. The top 2 rows of the tableshow the amino acid residues within mouse Fbs that correlate to humanFbs.

FIG. 9B demonstrates that several Fbs mutants possess increased affinityto fetuin, a complex-type N-glycan linked glycoprotein. Fetuin wasconjugated to Affigel-15 beads and then incubated with wtFbs orindividual Fbs mutant proteins. The top panel shows the amount of boundand eluted Fbs protein by -PAGE analysis. Image J software was used toquantify the amount of bound Fbs protein in order to generate the bargraph. Binding was carried out in low salt buffer (50 mM ammoniumacetate pH 7.5).

FIG. 9C demonstrates the binding properties of combinatorial mutants ofFbs. The panel labeled “Pulldown by Fetuin” shows the amount of wtFbs orvariant Fbs that binds to Affigel-Fetuin beads. The PPR mutant wascombined with the YR mutant to generate quintuple mutant PPRYR, whichshows the highest affinity to fetuin in low salt buffer (50 mM ammoniumacetate pH 7.5). The Lysate panel shows that equivalent amounts of eachFbs protein were present in the respective cell lysates incubated withAffigel-Fetuin beads.

FIGS. 10A-10C show the binding characteristics of 2 selected Fbs mutantsin comparison to wtFbs.

FIG. 10A shows that the GYR mutant and PPRYR mutant both possessincreased affinity to fetuin and no change in affinity to RNaseB in lowsalt buffer (50 mM ammonium acetate pH 7.5). In high salt conditions(2000 mM ammonium acetate pH 7.5), the GYR mutant shows the highestaffinity to both fetuin and RNaseB, which were added simultaneously toFbs beads.

FIG. 10B shows that an equivalent amount of each SNAP-tagged testprotein was conjugated to benzylguanine (BG) beads. FT indicates theamount of test protein in the flow-through after conjugation.

FIG. 10C is a table which summarizes the relative affinities of wtFbs,the GYR mutant and the PPRYR mutant to fetuin and RNaseB. A ratio of 1or near to 1 indicates that binding affinity to fetuin (a complex-typeN-glycan linked glycomolecule) and to RNaseB is unbiased. The values inthe table were calculated according to ImageJ quantification of the bandintensities in FIG. 10A.

FIG. 11 shows the relative binding affinities of wtFbs, the GYR mutantand the PPRYR mutant to SGP-TMR. Each Fbs protein was conjugated to BGbeads and then incubated with SGP-TMR. After incubation at 4° C. for 1hour, the Fbs beads were centrifuged and the unbound SGP-TMR in thesupernatant was measured. Recovery (%) is the amount of boundglycomolecule and was calculated by measuring fluorescence of the inputsolution and then subtracting the amount of fluorescence in thesupernatant. The GYR mutant possesses superior binding affinity to thissialylated substrate in low salt buffer (50 mM ammonium acetate pH 7.5).

DETAILED DESCRIPTION

Described herein are methods and compositions described for specificallycapturing N-glycan and/or N-glycan linked glycomolecules in asubstantially unbiased manner without the need for irreversible changesto the N-glycan composition or structure and without the need formultiple binding reagents. Advantageously, the captured N-glycan linkedglycomolecules can be released under conditions that are compatible withstandard downstream analytical protocols.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe pertinent art. Embodiments described herein may include one or moreranges of values (e.g., size, concentration, time, temperature). A rangeof values will be understood to include all values within the range,including subset(s) of values in the recited range, to a tenth of theunit of the lower limit unless the context clearly dictates otherwise.

As used herein, the articles “a”, “an”, and “the” relate equivalently toa meaning as singular or plural unless the context dictates otherwise.

A “glycomolecule” here refers to a non-carbohydrate entity such aslipids, proteins or other biological or non-biological molecules towhich is attached one or more carbohydrate moieties.

Examples of glycomolecules are glycoproteins which include proteins,polypeptides or peptides to which is linked one or more glycans. Theprotein or a proteolytically cleaved glycoprotein may have a sizesuitable for mass spectrometry analysis. For example, a proteolyticallycleaved glycoprotein may have an amino acid composition of less than 500amino acids for example, less than 400 amino acids, for example, lessthan 300 amino acids, for example, less than 200 amino acids. In oneembodiment, the proteolytically cleaved glycoprotein has less than 150amino acids.

Glycosylation refers to the covalent attachment of a glycan to apolypeptide, lipid, polynucleotide or another glycan. The terms“glycosylated peptide” “glycosylated polypeptide” or “glycosylatedprotein” can be used interchangeably with “glycopeptide,”“glycopolypeptide” or “glycoprotein.”

Wherever N-glycan linked glycomolecules react with the binding reagent,it is expected that the binding reagent will react in a similar mannerwith N-glycans that have been cleaved from the N-glycan linkedglycomolecules.

A “glycan” refers to a carbohydrate entity that has (a) apentasaccharide core structure with a GlcNAc at its reducing end capableof covalent linkage to the amine group on the side chain of asparaginein a protein, polypeptide or peptide (N-glycan); (b) an oligosaccharidewith a GalNAc capable of forming a covalent bond with a hydroxyl groupon the side chain of a serine or threonine (O-glycan); or (c) othercarbohydrate entities capable of covalent linkage to a protein or lipid.

An N-glycan may have a fucose at the reducing end, linked to the GlcNAc.In one embodiment, N-glycans may have one or more sialic acids at thenon-reducing ends of the pentasaccharide core. In another embodiment,the pentasaccharide core may have alternative branches of saccharides oradditional saccharides extensions on the core branches. One or moresialic acids may be found on these alternative branches or extensions ofthe glycan.

The sialic acids including variants thereof impart charge to the glycanshence the term “charged”.

The attachment of glycans to proteins, polypeptides or peptides isreferred to herein as glycan-linked glycomolecules. Those glycomoleculesthat are enriched by embodiments described herein contain N-linkedglycans but may also contain O-linked glycans and/or other carbohydrateentities. These are “N-glycan linked glycomolecules”.

The terms “enriched” or “enriching” refers to a reduction in complexityof a mixture. Here the representation of N-glycan linked glycomoleculesmay be increased by more than 50%, 60%, 70%, 80%, 90%, 100%, 150%, 200%,or more compared to an un-enriched solution.

The terms “substantially unbiased” refer the capacity of the bindingreagent to bind to an N-glycan either alone or linked to a protein,peptide or polypeptide regardless of its composition or linkageposition. The term “substantially” refers to at least 50%, 60%, 70%,80%, 85%, or 90%. One example shows substantially unbiased bindingresulting in 65% recovery and a second example shows about 90% recoveryof a charged N-glycan linked glycoprotein based on a comparison withrecovery of an uncharged N-glycan linked glycoprotein.

The term “cleavage enzyme” refers to enzymes for cleaving proteins,polypeptides or peptides such as proteases; enzymes that cleave glycansfrom glycomolecules such as amidases such as PNGase; enzymes that cleavebetween adjacent sugar residues such as exoglycosidases orendoglycosidases and enzymes that cleave contaminating nucleic acidsfrom cell lysates such as DNAases and RNases. Further examples ofcleavage enzymes include trypsin, endoproteinase GluC, endo proteinaseAspN, proteinase K, Factor Xa protease, IdeS, IdeE, enterokinase, furin,endonuclease S, neuraminidase, PNGase Ar, PNGase A, PNGase F,O-glycosidase, endo D, endo F, endo F1, endo F2, endo F3, endo H, endoS, beta1-3 galactosidase, beta1-4 galactosidase, alpha1-3,6galactosidase, beta-N-acetylglucosaminidase,alpha-N-acetylgalactosamindiase, beta-N-acetylhexosaminidase, alpha1-2,3mannosidase, alpha1-6 mannosidase, neuraminidase, alpha2-3neuraminidase, alpha1-2 fucosidase, DNase, RNase H, or a combinationthereof.

An “immobilization module” refers to a protein, nucleic acid, syntheticmolecule or other chemicals capable of coupling with a binding reagentfor purposes of immobilizing the N-glycan linked glycoproteins byaffinity binding. Examples of immobilization modules are known in theart and include: maltose-binding proteins (MBPs) (U.S. Pat. Nos.5,643,758 and 7,825,218; 7,883,867, and 8,623,615), SNAP-TAG® (NewEngland Biolabs, Ipswich, Mass.) (utilizing AGT, see U.S. Pat. Nos.7,939,284, 8,367,361, 7,799,524, 7,888,090, 8,163,479, and 8,178,314),CLIP-TAG™ (utilizing ACT, see U.S. Pat. Nos. 8,227,602 and 8,623,627),inteins (U.S. Pat. Nos. 5,496,714, 5,834,247, 6,521,425, 7,157,224,6,849,428, 7,001,745, 6,858,775, and 7,271,256), chitin-binding proteins(U.S. Pat. Nos. 6,897,285, 7,060,465, 6,984,505, and 6,987,007), biotin,streptavidin, antibodies, or Flag-tags.

“Labeling” of N-glycans or N-glycan linked glycomolecules includeslabeling the N-glycans and/or labeling the non-carbohydrate entity.Labeling of glycans can be achieved using any label known in the artincluding: 2-AB, (Anthranilamide, Anthranilic acid amide), Anthranilicacid (2AA) (2-Aminobenzoic acid), 2-Aminopyridine (2-AP)(2-Pyridinamine, 2-Pyridylamine), 2-aminonaphthalene trisulfonic acid(ANTS), 1-aminopyrene-3,6,8-trisulfonic acid (APTS),1-phenyl-3-methyl-5-pyrazolone (PMP), 2,6-Diaminopyridine (DAP),4-Aminobenzamidine (4AB), 7-Amino-4-methylcoumarin (AMC), procainamide,RapiFluor-MS™ (Waters, Milford, Mass.), aminoxyTMT™ (Life Technologies,Carlsbad, Calif.), and IMS (Prozyme, Hayward, Calif.) (also see Ruhaak,et al., Anal Bioanal Chem. 397, 8, 3457-3481 (2010); Kalay, et al., AnalBiochem., 423, 1, 153-62 (2012); and Radoslaw, et al., AnalyticalBiochemistry, Available online 12 Jun. 2015). Labeling ofnon-carbohydrate entities may occur for fusion molecules throughlabeling an immobilization module (for example, modified benzyl guaninesubstrate for AGT) or by direct labeling of the non-carbohydrate entityor by utilizing a nucleic acid aptamer labeled with a tag and capable ofassociating with the non-carbohydrate entity.

A “binding reagent” is described in more detail below and refers to areagent that specifically and selectively binds N-glycan linkedglycomolecules. In one embodiment, the binding reagent includes a SBD ofa polypeptide such as for example, the SBD of F-box sugar domain (Fbs).In one aspect, the binding reagent comprises a consensus sequence of apolypeptide that has 70%, 75%, 80%, 85%, 90%, 95%, 99% or 100% sequenceidentity to SEQ ID NO: 1.

(SEQ ID NO: 1) F₁₁₃YFLSKRRRNLXXNXCGXXXLXXWXXVEXGGDGWXXEXLPGDXGXXFXXXXXVXKXFXXSXEWCRKXQXXDLXAEGYWEELLDXXQPXXXXKDWYXGRXDAGXXYELXVKLLSXXEXVLAEXXXXXXAXPXXXXXXXWXXISXTFXXYGPGVRXXRFXHXGQDXXXWKGWXGXRXTNSSVXVXP,wherein X is any amino acid.

X may be selected from the following amino acids in Table 1:

TABLE 1 X_(12,) may be X₅₆ may be X₁₁₄ may be X₁₄₆ may be L or I Y or FV or E T or S X_(18,) may be X₅₈ may be X₁₁₅ may be X₁₅₄ may be E or D Aor V H or N F or Y X_(19,) may be X₅₉ may be X₁₁₇ may be X₁₅₅ may be Eor D S or T D or N V or I X_(20,) may be X₆₁ may be X₁₂₂ may be X₁₅₈ maybe E or D F or Y F or Y E or Q X_(22,) may be X₆₇ may be X₁₂₄ may beX₁₆₀ may be E or Q A or S S orT A or G X₂₃, may be X₆₉ may be X₁₂₅ maybe X₁₆₅ may be G or H V or I G or E V or L X_(26,) may be X₇₀ may beX₁₂₆ may be X₁₆₆ may be E or D V or I Q or T Y or F X₂₉ may be X₈₈ maybe X₁₂₇ may be X₁₇₁ may be H or N A or K V or I Y or F X₃₅ may be X₈₉may be X₁₂₉ may be X₁₇₃ may be R or K I or V V or I A or V X₃₆ may beX₉₀ may be X₁₃₂ may be X₁₇₅ may be V or I V or M D or E V or M X₃₈ maybe X₉₁ may be X₁₃₆ may be X₁₈₁ may be D or E V or A A or G W or T X₄₆may be X₉₆ may be X₁₃₇ may be X₁₈₃ may be E or D S or A G or S Q or EX₅₀ may be X₁₀₃ may be X₁₄₀ may be E or D C or S E or Q X₅₁ may be X₁₀₄may be X₁₄₃ may be E or D L or V H or Y

In some embodiments, one example of SEQ ID NO: 1 is a polypeptidecomprises an amino acid sequence at least 70%, 75%, 80%, 85%, 90%, 95%,99% or 100% sequence identity to:

(SEQ ID NO: 2) F₁₁₃YFLSKRRRNLLRNPCGEEDLEGWCDVEHGGDGWRVEELPGDSGVEFTHDESVKKYFASSFEWCRKAQVIDLQAEGYWEELLDTTQPAIVVKDWYSGRSDAGCLYELTVKLLSEHENVLAEFSSGQVAVPQDSDGGGWMEISHTFTDYGPGVRFVRFEHGGQDSVYWKGWFGARVINSSVWVEP₂₉₆.This truncated protein corresponds to the SBD of an N-glycan bindingubiquitin ligase.

Sequence identity can be determined by those of skill in the art usingstandard techniques, including sequence algorithms like BLAST (Altschul,et al., J. Mol. Biol. 215, 403-410 (1990); Altschul, et al., Nuc. Ac.Res. 25, 3389-3402 (1997) and FASTA, Pearson and Lipman, PNAS 85, 8,2444-2448 (1988)).

Variants of SEQ ID NO: 2 have been generated to obtain greater bindingaffinity to N-linked glycan glycomolecules or N-glycans than thetruncated non mutated Fbs protein. An added benefit is that the variantsdo not rely on a high salt binding buffer for optimal binding toN-glycans and N-linked glycan glycomolecules which is the preferableoption for the wild type protein. Examples of advantageous mutationsites are provided in Table 2. Table 2 describes 5 different mutationpositions: 154, 155, 156, 173 and 174 where the numbering corresponds toSEQ ID NO: 2. As shown in the table, mutations assigned to thesepositions are for D154P, S155A, S155G, S155P, G156S, G156R, F173Y andE174R. It was shown that the mutations individually or in variouscombinations were capable of improving binding. The combinations mayinclude single mutations, double mutations, triple mutations, 4mutations, or 5 mutations selected from those in table 2. The benefitsof single mutations and a plurality of mutations are illustrated inFIGS. 9A-9C and FIG. 10A-10C and FIG. 11.

Binding reagents exemplified by SEQ ID NO: 1 and SEQ ID NO: 2 may befused to an immobilization module (see for example SEQ ID NO: 4) to forma fusion protein (SEQ ID NO: 3). Other examples of immobilizationmodules include peptides with at least 80%, 85%, 90%, 95%, 98% or 99%sequence identity with SEQ ID NO: 4.

The improved binding of mutant Fbs to N-glycans and N-glycan linkedglycomolecules suggests that the mutants may be used to reduce BACE1levels and reduced synaptic deficits in Alzheimer patients (see Ohtsubo,et al., Sugar Chains: Decoding the function of Glycans, p. 9 (2015)).Another glycoprotein that is involved in Alzheimer's disease is theamyloid precursor protein (APP) whose cleavage products, particularlyamyloid-β, accumulate in Alzheimer disease (AD). APP is present atsynapses and is thought to play a role in both the formation andplasticity of these critical neuronal structures. Wild type Ubiquitinligase Fbxo2 is believed to regulate APP levels and processing in thebrain and may be a therapeutic approach to modulating AD pathogenesisand associated symptoms. It is proposed here that Fbs variants describedherein would improve the efficacy of this beneficial effect.

Another use for Fbs variants is as an antiviral agent. Many viruses haveglycomolecules on their surface such as Ebola virus, herpes virus andinfluenza virus and many others. Fbs variants are expected toefficiently interact with envelope glycoproteins to prevent infection ofnew cells.

Another use for Fbs variants is for counteracting hearing lossassociated with aging and deterioration of inner ear homeostasis.

The role of glycoproteins in health and disease is extensive althoughnot yet well understood. Fbs variants that bind to glycomolecules inmembranes of cells may prove to be very useful therapeutic agents andmay further increase the understanding of the role of glycoproteins fordiseases where glycomolecules from host and/or pathogen are incorrectlyfolded in viva Fbs variants may be substituted for other variants havinga consensus sequence (SEQ ID NO: 1) or binding reagents that mimic thefunction of these variants.

TABLE 2 Variants having increased binding affinity for N-glycans andN-glycan linked glycomolecules. a.a. position 158 159 160 177 178 Mousewt D N G F E FbS1 a.a. position 154 155 156 173 174 Human Fbs wt D S G FE S155A D A G F E S155G D G G F E PPG P P G F E PPS P P S F E PPR P P RF E YR D S G Y R S155G + YR D G G Y R PPR + YR P P R Y R

In some embodiments, the binding reagent is a fusion protein. Forexample, any of the above polypeptide can be fused to one or morereagents that can serve as a label, tag, or immobilization module.Examples of labels include fluorescent, chemiluminescent, or radioactivelabels.

The fusion protein may be immobilized on a two dimensional matrix suchas filter paper or a three dimensional matrix such as a polymer thatforms a fabric or a polymer or a surface of a device such as amicrofluidic device.

FIG. 1 shows the work flow that results in an enriched N-glycan linkedpeptide that can be analyzed by mass spectrometry or by other analyticaltechniques such as chromatography, electrophoresis, nuclear magneticresonance spectrometry and fluorescence-mediated detection, or acombination thereof.

A glycomolecule sample can be derived from any source such as forexample, a biological fluid such as blood, serum, urine or mucose, afermentation media containing secreted cell products, cell lysate from abiopsy or fermentation may be analyzed by a workflow as demonstrated inFIG. 1. N-glycan linked glycomolecules can be in or from a sample of arecombinantly synthesized, chemically synthesized, an environmentalsample (e.g., soil, water, air, surface swab). Such sources includeanimals (including humans, domesticated and non-domesticated animals),avian, insect, aquatic, bacteria, viruses and plants.

In some embodiments, the N-glycan linked glycomolecule sample is acomplex mixture. For example, the complex mixture may contain multipleN-linked glycan glycomolecules as well as other non-N-glycan linkedglycomolecules. For example, when the sample is a body fluid such asserum or plasma, the sample contains not only many different N-linkedglycan glycomolecules, but also non-carbohydrate entities that do nothave N-glycans such as lipids, DNA, RNA, organelles, whole cells andfragments of cells.

In some embodiments, the N-linked glycan glycomolecules can be native ordenatured. For example, the N-linked glycan glycomolecules can bedenatured by heat treatment, exposure to a reducing agent or otheragents that tend to disrupt the native conformation.

Embodiments of the binding reagents provided herein selectively andreversibly binds to all N-linked glycan glycomolecules, because thebinding reagents target a core pentasaccharide N-glycan structure at thereducing end of the glycan irrespective of whether the N-glycan ischarged or uncharged. Charged N-glycans may contain a sialic acid linkedto the terminal mannose on the forked structure shown in FIGS. 1, 5, 7C.Examples of N-glycans, include high mannose, hybrid, and complex-typeN-glycans. More specifically, the pentasaccharide isMan(α1-3)(Man(α1-6))Man(β1-4)GlcNAc(β1-4)GlcNAc. In some cases, a fucosemay also be found attached to the ultimate GlcNAc. Surprisingly, asdescribed in Example 5 and shown in FIG. 3, that binding of the bindingreagent to the core structure is not affected by the presence of fucoseon the core structure. Embodiments of the binding reagents are capableof binding N-glycans and/or N-glycan linked glycomolecules in asubstantially unbiased manner. This is an important feature as it avoidsunder-representing or even missing N-linked glycan glycomolecules in amixture. The Examples illustrate effective binding of charged N-linkedglycan glycomolecules (see Examples 7, 9, 10, 11) under suitable saltconditions and this binding is substantially unbiased (see Examples 8,12, 13). Prior art methods preferentially capture either charged oruncharged N-glycans, but not both.

The binding reagents described herein do not include lectins that bindGlcNAc in any position of the glycan, including the branched antennae ofcomplex and hybrid glycans and does not target the core structure.

For cell lysates, it may be desirable to degrade DNAs and RNAs and toutilize detergents to liberate glycomolecules. If the glycomolecule ofinterest is an antibody, it may be desirable to bind the sample toprotein A in order to obtain an enriched antibody sample. For solubleenzymes, it may be desirable to precipitate cellular material leavingthe glycomolecules in the supernatant.

This step may be followed by an enzyme cleavage step. In FIG. 1, theenzyme cleavage step is accomplished by a protease which cleavesproteins into peptides. Some of peptides will have N-linked glycans.

At this stage, an attempt to perform mass spectrometry will generate avery complex profile as illustrated in FIG. 1. However, enrichment canbe achieved if the cleavage products are reacted to a binding reagentthat selectively binds all N-glycan linked glycopeptides. To achievethis, it was found that a high salt binding buffer substantiallyeliminated bias when a SBD of a binding protein (Fbs) was utilized as abinding reagent to selectively bind N-glycans linked to theglycomolecules. It was subsequently found that synthetic variants ofthis binding protein did not require high salt conditions in the bindingreaction.

In some embodiments, the high salt concentration in a buffer, sometimesreferred to herein as a first buffer, is at least or about 500 mM, 750mM, 1 M, 1.5 M, 2 M, 2.5 M, or 3 M. Suitable salts include ammoniumacetate, ammonium chloride, ammonium sulfate, calcium acetate, calciumchloride, magnesium acetate, magnesium chloride, magnesium sulfate,potassium acetate, potassium chloride, potassium sulfate, sodiumacetate, sodium chloride, sodium sulfate. The Examples describe the useof such suitable salts, including sodium chloride and ammonium acetate.Unexpectedly, high salt does not disrupt the binding between the bindingreagent and N-glycan linked glycomolecules.

High salt conditions are generally accepted in the prior art for use indisrupting the binding interaction between binding partners. Incontrast, high salt conditions are shown herein to not disrupt thebinding and instead improve binding of the binding reagent to N-glycanlinked glycomolecules. Example 6 details a titration curve showingincreased binding with increasing ionic strength of the buffer. Withoutwishing to be bound by theory, the high salt conditions may neutralizeany charged N-glycan linked glycomolecules. This facilitatessubstantially unbiased binding of the reagent to both charged anduncharged N-glycan linked glycomolecules.

It was also shown that the binding reagents could effectively bindN-glycan linked glycoproteins when immobilized. Immobilization shown inFIG. 1 results from forming a fusion protein between the SBD of Fbs orvariants thereof and a modified DNA repair enzyme called AGT.Optionally, the fusion protein may form covalent linkages with benzylguanine derivatives that may be present in solution or form coatings ona solid, semi-solid or porous surface such as a bead or other polymermatrix such as a well, column, plate or a microfluidic device. Example 1illustrates the use of a binding protein that is a fusion proteinbetween SNAP-tag and Fbs and immobilization of the binding reagent onbeads.

Example 1 and FIG. 1 show how the methodology is efficient for improvingmass spectrometry results for purposes of identifying the site ofattachment of an N-glycan on a protein, the composition of the N-glycanand/or the properties of the peptide to which the N-glycan is attached.Once the N-glycan linked glycopeptides are bound to the binding reagentand the non-binding material washed away, the N-glycan linkedglycopeptides can be efficiently eluted using a second buffer. Thisbuffer utilizes an organic solvent, acidic pH or low ionic strength forefficient release of bound N-glycan linked glycomolecules. Example 14demonstrates suitable examples of different reagents for releasingcaptured N-glycan linked glycomolecules. In some embodiments, the bufferis volatile preferably comprising 50% v/v acetonitrile in water or thebuffer is 1% acetic acid. Other reagents may be used such as water,dichloromethane, dichloroethane, pentahydrofuran, ethanol, propanol,isopropanol, methanol, nitromethane, toluene, DMSO, acetic acid, formicacid or a mixture thereof. Preferably SDS is not used as it interfereswith mass spectrometry.

The N-glycan linked glycomolecules can be readily analyzed. For example,the N-glycan linked glycomolecules can be analyzed by mass spectrometry,chromatography, electrophoresis, nuclear magnetic resonance spectrometryand fluorescence-mediated detection, or a combination thereof. See anyone of Examples 2-4, 6, 8, 9, 11-17 for examples of such analyses. Theanalysis of the N-glycan linked glycomolecules can characterize thestructure of one or more N-glycans, identify the N-glycosylation linkagesite on an N-glycan linked glycopeptide or N-glycan linked glycoprotein,quantify the amount of one or more different N-glycans, or anycombination thereof. Such analyses are useful in the identification ofN-glycan linked glycomolecule biomarkers, may assist in diseasediagnosis, or be used to monitor disease progression or treatment.

The workflow shown in FIG. 1 is also suited for the analysis ofN-glycans that may have been cleaved from proteins using a PNGase.However, such an analysis cannot provide information directly about thelinkage site of N-glycan on a peptide although it can provideinformation on the amount and composition of the N-glycans.

The enrichment of N-glycans by binding to an immobilized binding reagentis not limited to the use of Fbs or to the use of AGT. Advantageously,the methods and reagents described herein are substantially unbiased incapturing all N-glycan linked glycomolecules, whether an N-glycan or anN-glycan linked glycopeptide or N-glycan linked glycoprotein, andwhether the N-glycans are charged or uncharged. Furthermore, the methodsare readily compatible with downstream analytically methods, such asmass spectrometry, which are sensitive to interference by reagents likedetergents.

The methods and compositions described herein are non-natural. Forexample, the binding reagent is non-natural. Where the term “wild type”is used with respect to the binding agent, it refers to an isolateddomain derived from a wild type protein, namely the SBD that does notitself occur naturally. Hence the wt ubiquitin ligase SBD of SEQ ID NO:1 and 2 are non-natural. Any variant of this polypeptide is alsounnatural. The variants were identified by experimentation in vitro andare not known to occur in nature either as an isolated domain or as partof a larger protein. Where the wild type is fused to an immobilizationmodule, this is also an unnatural construction and is not known to occurin nature. The methods described herein do not mimic and cannot beconstrued as natural.

SEQ ID NO: 1 has been given an amino acid number of 1 for the firstamino acid in the consensus polypeptide sequence. In SEQ ID NO: 2, thefirst amino acid is labeled as 113 which corresponds to amino acid 1 inSEQ ID NO: 1. In fact it is a truncated version of the human fbs1protein in which the first 112 amino acids have been removed. SEQ ID NO:2 is one example of the consensus sequence described by SEQ ID NO: 1 andis not intended to be limiting.

(SEQ ID NO: 3) MGSSHHHHHHGDKDCEMKRTTLDSPLGKLELSGCEQGLHEIKLLGKGTSAADAVEVPAPAAVLGGPEPLMQATAWLNAYFHQPEAIEEFPVPALHHPVFQQESFTRQVLWKLLKVVKFGEVISYQQLAALAGNPAATAAVKTALSGNPVPILIPCHRVVSSSGAVGGYEGGLAVKEWLLAHEGHRLGKPGLGPAAIGAPGSGSGSPAGCQQEGLVPEGGVEEERDHWQQFYFLSKRRRNLLRNPCGEEDLEGWCDVEHGGDGWRVEELPGDSGVEFTHDESVKKYFASSFEWCRKAQVIDLQAEGYWEELLDTTQPAIVVKDWYSGRSDAGCLYELTVKLLSEHENVLAEFSSGQVAVPQDSDGGGWMEISHTFTDYGPGVRFVRFEHGGQDSVYWKGWF GARVTNSSVWVEP

The sequence in bold is a linker sequence. The sequence preceding thelinker sequence is AGT variant (immobilization module). The sequencefollowing the linker sequence is SEQ ID NO: 2 and could be a variantthereof.

(SEQ ID NO: 4) MGSSHHHHHHGDKDCEMKRTTLDSPLGKLELSGCEQGLHEIKLLGKGTSAADAVEVPAPAAVLGGPEPLMQATAWLNAYFHQPEAIEEFPVPALHHPVFQQESFTRQVLWKLLKVVKFGEVISYQQLAALAGNPAATAAVKTALSGNPVPILIPCHRVVSSSGAVGGYEGGLAVKEWLLAHEGHRLGKPGLGPA

SEQ ID NO: 4 is the immobilization module (AGT variant) as used for thefusion protein in SEQ ID NO: 3.

All references cited herein including U.S. application Ser. No.15/320,911, filed Dec. 21, 2016; International Application No.PCT/US2015/37960 filed Jun. 26, 2015; and U.S. Provisional Ser. No.62/020,335 filed Jul. 2, 2014 are incorporated by reference.

EXAMPLES

The following experiments refer to various buffers. These are asfollows:

Trypsin digestion buffer: 50 mM ammonium bicarbonate.

The first buffer or binding buffer may be a high salt buffer, high saltvolatile buffer, a low salt buffer or a low salt volatile buffer.Volatile buffers are preferred for mass spectrometry.

High salt buffer: 2 M NaCl, 20 mM Tris-HCl, 1 mM EDTA, pH 7.5;

High salt volatile buffer: 2 M ammonium acetate, pH 7.5 (for use in massspectrometry);

Low salt buffer: 50 mM NaCl, 20 mM Tris-HCl, 1 mM EDTA, pH 7.5;

Low salt volatile buffer: 50 mM ammonium acetate, pH 7.5 (for use inmass spectrometry);

Dialysis buffer: may be the same as the first buffer;

The second buffer is 50% acetonitrile v/v in water.

Example 1: Enrichment of N-Glycan Linked Glycomolecules from a ComplexMixture (FIG. 1)

Protein or peptide based glycomolecules within a complex sample may becharacterized using Fbs to accomplish enrichment of N-glycan linkedglycopeptides. For example, the method may applied to biological samplessuch as serum, urine, cell lysates, or other body fluids. The complexmixture containing N-glycan linked glycoprotein, non-glycosylatedprotein and other biomolecules is first digested with a protease such astrypsin in 50 mM ammonium bicarbonate. The protease is optionallyremoved by filtration or other means such as immobilization. A proteaseinhibitor such as phenylmethanesulfonyl fluoride (PMSF) or trypsininhibitor is added to inhibit proteolysis. The glycopeptide and peptidemixture is then incubated with Fbs immobilized on beads or immobilizedon another surface in either a low salt or high salt buffer. While theN-glycan linked glycopeptides bind to immobilized Fbs, O-linkedglycopeptides and non-glycosylated peptides or proteins will remain insolution. One or more washing steps are carried out to remove materialthat is not bound to immobilized Fbs. The wash buffer is typically thesame as the buffer used for binding. The bound N-glycan linkedglycopeptides are then eluted using a mass spectrometry (mass spec)friendly solvent such as 50% acetonitrile in water. The enriched sampleis analyzed by mass spec (Nilsson, et al., Nature Methods, 6, 809-813(2009)) to reveal the identity and relative quantity of N-glycan linkedglycopeptides (see box labeled “after selection”).

Example 2: Over-Expression of an Exemplary Binding Reagent andImmobilization to a Solid Support

An example of a binding reagent shown in FIG. 1 is Fbs fused to theSNAP-tag protein. SNAP-tag substrates are derivatives of benzylpurinesand benzylchloropyrimidines. In the conjugation reaction, thesubstituted benzyl group of the substrate is covalently attached to theSNAP-tag. SNAP-Fbs expresses at a high level as a soluble protein in E.coli using pSNAP-tag(T7)-2 vector (New England Biolabs, Ipswich, Mass.).

SNAP-Fbs is efficiently conjugated to BG beads (SNAP Capture Pull-Downresin (New England Biolabs, Ipswich, Mass.): SNAP-Fbs protein is reactedwith BG beads at 4° C. overnight in a DTT-containing buffer (4 mM DTT,20 mM tris-HCl pH 7.5, 1 mM EDTA, 50 mM NaCl). After the conjugationreaction, SNAP-Fbs beads are washed with 40× bead volume of low saltbuffer.

Example 3: N-Glycan-Specific Binding by the Binding Reagent (FIG. 2A)

N-Glycan-specific binding of the binding reagent was demonstrated usingRNase B. RNase B is a glycoprotein with an N-linked high mannose glycan.Thirty micrograms RNase B was denatured by boiling in 0.5% SDS and 40 mMDTT. After boiling, 1% NP-40 was added. The denatured RNase B was eitheruntreated or treated with PNGase F (for 1 hour at 37° C. in 50 mM sodiumphosphate, pH 7.5) to remove N-glycans. The untreated and treatedsamples were incubated with SNAP-Fbs conjugated to BG beads(SNAP-Capture Pull-Down resin) at 4° C. for 1 hour in low salt buffer.Fbs beads were centrifuged, then washed twice with the low salt buffer,and bound proteins on the beads were eluted in 1×SDS gel loading buffer(New England Biolabs, Ipswich, Mass.) and analyzed by SDS-PAGE. Lane 1is control untreated RNase B containing N-glycan that was not exposed toFbs beads. Lane 2 is an RNase B sample pre-treated with PNGase F andthen incubated with Fbs beads. Thus N-linked glycoprotein is not boundand eluted. Lane 3 is untreated RNaseB which was bound and eluted fromFbs beads.

Example 4: The Binding Reagent Efficiently Binds to N-Glycan LinkedGlycomolecules with Complex N-Glycans (FIG. 2B)

Efficient binding of the binding reagent to N-glycan linkedglycomolecules with complex N-glycans was demonstrated using human IgG(Bethyl Laboratories, Montgomery, Tex.). Binding depends on the presenceof the N-glycan. Human IgG is a glycoprotein with an N-linked glycan inthe heavy chain (at Asn297) with complex glycan structure, and typicallyno N-linked glycans are present in the light chain. The same conditionsdescribed in Example 3 were used to assay 50 μg human IgG binding toFbs. Lane 1 is a control showing the IgG without any treatment. Lanes 3and 4 show that only the heavy chain is bound by Fbs beads and only whenglycan is present (Lane 4). In lanes 2, 3 and 4, the asterisk indicatesa small amount of SNAP-Fbs protein that is released from the beadsduring boiling in gel loading buffer to elute bound heavy chain. Thelight chain of IgG, on which there is no N-linked glycan, did not bindto Fbs beads. This is an example to show that Fbs binding is specificfor N-linked glycoproteins or glycopeptides.

Example 5: The Binding Reagent is Unaffected by Fucosylation of the CorePentasaccharide (FIG. 3)

Fbs binds to N-glycan and N-glycan with fucose modification at thereducing end of GlcNAc. Isothermal titration calorimetry was used toassay the interaction between Fbs and core structure glycans.Man3GlcNAc2 (M3N2) and Man3GlcNAc2 with (alpha 1-6) fucose modificationat the reducing end of GlcNAc (M3N2F) were used as binding substrates(both substrates were purchased from Prozyme, Hayward, Calif. SNAP-Fbswas dialysed against low salt buffer and the glycans were also dissolvedin the same dialysis buffer. A Nano ITC instrument (TA Instruments,Lindon, Utah) with 170 μl cell volume and 50 μl buret volume was used.Every 300 seconds, 2.97 μl of 90 μM glycan was automatically injectedinto 9 μM Fbs solution. The data was fitted using a single site bindingmodel (Nanoanalyze software (TA Instruments, Lindon, Utah)), and the Kdwas calculated. The affinity of Fbs for Man3GlcNAc2 (Kd M3N2=0.123±0.036μM) is very similar to the affinity of Fbs for Man3GlcNAc2 modified withfucose (Kd M3N2F=0.128±0.037 μM) indicating that N-glycan binds to Fbstightly and fucosylation of the first GlcNAc residue from the reducingend does not impair binding by Fbs.

Example 6: 2-AB Fluorophore Labeling at the Reducing End of M3N2Abolishes Binding by Fbs (FIG. 4)

When PNGase F is used to trim N-glycans away from the peptides orproteins, the terminal GlcNAc residue becomes available formodification. A common modification is fluorophore labeling using 2-ABdye. The primary amino group of the dye performs a nucleophilic attackon the carbonyl carbon of the acyclic reducing terminal residue to forma partially stable Schiff's base. The Schiff's base imine group ischemically reduced to give a stable labeled glycan. The conservedterminal GlcNAc residue is thus referred to as the “reducing end” of aglycan species. Upon 2-AB labeling, the carbohydrate ring structure isno longer capable of forming. Fbs binding to 2-AB labeled M3N2 wastested and the results are shown in FIG. 4. Fifty picomoles of 2-ABlabeled M3N2 in low salt buffer were incubated with SNAP-Fbs beads orSNAP control beads (SNAP Capture beads conjugated to SNAP-tag PurifiedProtein, New England Biolabs, Ipswich, Mass.). The majority of the 2-ABlabeled fluorophore was found in the flow-through and low salt bufferwash fractions in both samples indicating that 2-AB labeling disruptsthe interaction between glycan and Fbs. This consequence does not affectthe application of Fbs for N-glycan linked glycomolecule enrichment asthe 2-AB labeling step is an optional step downstream from theenrichment step.

Example 7: The Binding Reagent Effectively Binds Charged N-GlycansContaining Sialic Acids (FIG. 5)

Isothermal titration calorimetry analysis of SGP (Fushimi PharmaceuticalCo., Marugame, Japan) binding to SNAP-Fbs. SNAP-Fbs was dialysed againstlow salt buffer and SGP was also dissolved in the same low salt buffer.A Nano ITC instrument with 170 μl cell volume and 50 μl buret volume wasused. Every 300 seconds, 2.97 μl of 270 μM SGP was automaticallyinjected into 27 μM Fbs solution. The data was fitted using a singlesite binding model (Nanoanalyze software), and Kd was calculated. In lowsalt conditions, SNAP-Fbs binds a complex-type oligosaccharide chainwith two terminal sialic acid residues with a Kd=3.0±0.12 μM.

Example 8: Binding to Charged N-Glycan Linked Glycomolecules by theBinding Reagent is Substantially Unbiased in High Salt Buffer (FIG. 6A)

Fbs binding to complex N-glycan linked glycomolecules is substantiallyunbiased in a high salt buffer. SGP was labeled with twotetramethylrhodamine (TMR) fluorophores on the peptide portion vialysine residues according to the TMR protein labeling kit protocol(Genaxxon Bioscience, Ulm Germany). After TMR labeling, a fraction ofthe SGP-TMR sample was processed with neuraminidase (New EnglandBiolabs, Ipswich, Mass.) to remove terminal sialic acid residues. Twobinding conditions (LS, low salt buffer and HS, high salt buffer) weretested. The presence of sialic acid residues significantly reducesbinding in low salt conditions (compare column 1 vs. column 3) whereasbinding of sialylated SGP and asialo-SGP is greater than 65% in both lowsalt and high salt buffer (see column 2 and column 4). TMR Fluorescencein the supernatant was measured using a SpectraMax M5 fluorometer(excitation 555 nm and emission 595 nm with 590 nm cutoff). Binding ofSGP-TMR to Fbs (y-axis) is determined by subtracting the fluorescencemeasured in the bead supernatant from the input fluorescence.

Example 9: Binding of the Binding Reagent to Complex N-Glycan LinkedGlycomolecules is Enhanced by Increasing the Ionic Strength of theBinding Buffer (FIGS. 6A-6C)

In FIG. 6B, sialylglycopeptide labeled with two pentamethylrhodamine(PMR) fluorophores on the peptide portion (via lysines), was used as anN-glycopeptide substrate for a binding assay with Fbs beads. SGP-TMR wasdissolved in various solutions with increasing concentrations ofammonium acetate. Each sample was incubated with SNAP-Fbs beads for20-30 minutes at 4° C. The % SGP-TMR bound to the beads was determinedby measuring the fluorescence in the bead supernatant and thensubtracting this value from the input fluorescence. TMR Fluorescence wasmeasured using a SpectraMax M5 fluorometer (Molecular Devices,Sunnyvale, Calif.) (excitation 555 nm and emission 595 nm with 590 nmcutoff). Strikingly, binding of the Fbs to the complex N-glycan linkedglycomolecule was enhanced by increasing the ionic strength of thebinding solution in the range of 0 mM to 3000 mM ammonium acetate.

Example 10: High Salt Significantly Improves Binding of wtFbs to ComplexN-Glycan Linked Glycomolecules (FIG. 6C)

Isothermal titration calorimetry was employed to analyze the binding ofSNAP-wtFbs to SGP in solution. High salt buffer was tested as the commonbuffer for the binding reagent and the substrate SGP. Other conditionsfor the ITC experiment were the same as those listed in Example 7. Thesingle change in NaCl concentration from 50 mM to 2000 mM resulted in amuch greater affinity between wtFbs and SGP. The table in FIG. 6C showsthat high salt conditions result in a Kd=1.43±0.12 μM which is more thana 2-fold higher affinity compared to 3.0±0.12 μM in low salt buffer.

Example 11: Immobilized Fbs Binding Reagent Facilitates Enrichment ofN-Glycan Linked Glycomolecules from a Complex Sample (FIG. 7A)

Fbs beads were used to capture and enrich N-glycopeptides from a complexmixture. The complex mixture was a tryptic digest of RNase B spiked withSGP to serve as a complex, sialylated glycopeptide. RNase B containsseveral non-glycosylated peptides and several different high mannoseN-glycopeptides (labeled in the enlargement as: M5N2, M6N2). One hundredeighty-five micrograms RNase B was digested with 4 micrograms trypsin(New England Biolabs, Ipswich, Mass.) in trypsin digestion buffer andthen filtered with a 10 kDa Microcon 10 centrifugal filter unit(Millipore, Billerica, Mass.) to remove trypsin and any undigested RNaseB. PMSF (1 mM final concentration) was added to inhibit any residualtrypsin. 100 micrograms SGP was added to the RNase B digest. Thenimmobilized binding reagent, SNAP-Fbs beads in this example, was mixedwith the complex mixture and incubated at 4° C. for 1 hour in thepresence of either low salt volatile buffer or high salt volatilebuffer. The beads were washed with low salt volatile buffer or high saltvolatile buffer, respectively. The bound N-glycopeptides were elutedusing 50% acetonitrile, then lyophilized and analyzed by mass spec(LC-MS method). The TIC and enlargement demonstrates wild typeFbs-mediated binding and enrichment of N-glycopeptides from a complexmixture. The dotted line indicates the chromatogram of the input mixturewithout enrichment. The solid black line indicates the chromatogram ofthe high salt enrichment sample (HS enrichment=2M ammonium acetate pH7.5). The solid gray line indicates the chromatogram of the low saltenrichment sample (LS enrichment=50 mM ammonium acetate pH 7.5). Theenlarged box focuses on the glycomolecules that elute between 20-22minutes whereas the non-glycosylated peptides elute before 20 minutes.Strikingly, the non-glycosylated peptides were not enriched. Thus,N-glycopeptides present in the sample were selectively bound by thebinding reagent when using LS or HS enrichment.

Example 12: Quantification of the Selective Binding of N-Glycopeptidesfrom a Complex Mixture (FIG. 7B)

The different peptides and N-glycopeptides present in the complexmixture described in Example 11 were quantified. The extracted ionchromatogram results demonstrate that the binding reagent is selectivefor N-glycopeptides and does not capture non-glycosylated peptides fromthe complex mixture. Importantly, high salt volatile buffer improves theenrichment of SGP to the same level as non-sialylated M5N2 and M6N2.This demonstrates the universal ability of the wild-type Fbs bindingreagent to bind all N-glycan linked glycomolecules in a substantiallyunbiased manner.

Example 13: Wild-Type Fbs Enrichment of a Diverse Set of N-LinkedGlycopeptides is Substantially Unbiased in High Salt Volatile Buffer(FIG. 7C)

One hundred eighty micrograms human transferrin was digested with ninemicrograms trypsin in 50 mM ammonium bicarbonate and then filtered witha 10 kDa Microcon 10 centrifugal filter unit to remove trypsin and anyundigested transferrin. PMSF (1 mM final concentration) was added toinhibit any residual trypsin. The filter flow-through sample wasadjusted to 2M ammonium acetate, pH 7.5 (high salt volatile bufferconditions) and then incubated with SNAP-Fbs beads for 60 minutes at 4°C. Then the sample was washed three times with high salt volatile bufferand then eluted with 50% acetonitrile (in water). The sample waslyophilized to remove residual bicarbonate, ammonium acetate, water andacetonitrile.

The sample was resuspended in PNGase F reaction buffer according to theNew England Biolabs standard protocol. PNGaseF was added to releaseN-linked glycans from the digested glycopeptides. In this experimentN-glycans were released to reduce the complexity of the sample in orderto allow highly accurate determination of each glycan species. N-glycanswere labeled with 2-AB fluorophore (Prozyme, standard protocol) andanalyzed by LC-MS. The composition of human transferrin is known bythose skilled in the art so the glycans eluted in each peak can beinferred by using glycan standards. Mass analysis is also used toconfirm the mass of each glycan type. When using high salt volatilebuffer, the recovery of various types of complex N-linked glycomoleculeswas substantially unbiased within the range of 28.2 to 42.7%.

Example 14: N-Glycan Linked Glycomolecules are Reversibly Bound toImmobilized Binding Reagent and can be Readily Released (FIG. 8)

N-glycan linked glycomolecules reversibly binds to Fbs beads. Differentreagents were tested for releasing bound N-linked glycopeptides from Fbsbeads. 100 μl Fbs beads with bound SGP-TMR were collected, washed with300 μl low salt binding buffer once, and divided into three aliquots (30μl beads each aliquot). 100 ul of 2 M ammonium bicarbonate (NH₄HCO₃),50% acetonitrile, or 1% acetic acid were added to Fbs beads with boundSGP-TMR to elute SGP-TMR. The beads were centrifuged and supernatantswere collected. The solvent in the supernatant was removed by vacuumevaporation using a speedvac, and SGP-TMR was reconstituted in low saltbuffer. SGP-levels were measured as described in Example 9. FIG. 11Ashows that 50% acetonitrile readily released approximately 78% ofSGP-TMR in the first 100 μl elution. 1% Acetic acid releasedapproximately 65% of SGP-TMR. In contrast, less than 30% of boundN-glycan linked glycomolecules were released with 2 M ammoniumbicarbonate.

Example 15: Isolation of Fbs Mutants that Possess Improved Affinity toComplex N-Linked Glycomolecules (FIG. 9A-9C)

Fetuin, which contains sialylated complex N-linked glycans, served asthe complex N-linked glycomolecule in this example. Fetuin wasconjugated to Affigel 15 resin (Bio-Rad, Hercules, Calif.), and thendenatured by 6 M Guanidine HCL. After denaturation, the fetuin beadswere washed with 50 bead volumes of low salt buffer to remove GuanidineHCL. This immobilized denatured Fetuin was used to screen Fbs mutantsfor improved affinity to complex N-linked glycomolecules in low saltconditions in a fetuin pull-down assay. In this assay, cell lysatescontaining the same amount of wild type or mutant Fbs protein wereincubated with the same amount of fetuin beads at 4° C. for 1 hour.After incubation, the fetuin beads were washed three times with low saltbuffer. The bound Fbs proteins were eluted by 1× gel loading buffer, andanalyzed by SDS-PAGE. In the fetuin pull-down assay, the amount of Fbsmutant protein shown on the SDS-PAGE gel indicates the relative affinityof each Fbs mutant for binding to a complex N-linked glycomolecule. Fbsamino acid residue serine 155 was mutated to alanine to assess theeffect on the interaction with complex N-linked glycomolecules. Mutationof Ser155 to alanine was expected to reduce the affinity of theinteraction between Fbs and fetuin according to the published report ofTanaka et al. (Tanaka, et al. PNAS). Contrary to the expectation, theS155A mutant showed 1.5 times greater ability to be pulled down byimmobilized fetuin. Further investigation revealed that Fbs mutant S155Gshows 2.0-fold greater capacity to interact with fetuin as determined bythe fetuin pull-down assay.

To expand the search for high affinity Fbs mutants, plasmid display(Speight, et al., Chemistry & Biology, 8, 951-965 (2001)) was used toisolate Fbs mutants from a mutant library prepared by saturationmutagenesis of positions D154, S155, G156, F173 and E174 of human Fbs.Four additional Fbs mutants (designated as PPG, PPS, PPR, and YR in thetable FIG. 9A) were identified which possess higher affinity (up to 2.8fold increase) to the complex N-linked glycans on Fetuin (FIG. 9B). TheFbs mutant containing combined S155G and YR mutations (S155G+YR, alsotermed as GYR) or PPR and YR mutations (PPR+YR, also termed as PPRYR)possess even higher affinity (up to 4.5 fold increase) against complexN-linked glycans on Fetuin (FIG. 9C).

Example 16: Fbs GYR and PPRYR Mutants Show Substantially Reduced Bias inBinding to all Types of N-Linked Glycomolecule (FIGS. 10A-10C)

WT Fbs preferably binds to high mannose N-glycan linked glycomoleculessuch as RNase B, and possesses far lower affinity to sialyated complexN-glycan linked glycomolecules such as Fetuin. Fbs GYR and PPRYR mutantsshow significantly improved affinity to Fetuin (FIG. 9A-9C). The sameamount of wild-type Fbs, Fbs GYR mutant and PPRYR mutant proteins wereconjugated to SNAP capture resin using the conditions described inExample 2 (FIG. 10B). Binding to RNase B with high mannose N-glycan andFetuin with sialyated complex N-glycan linked glycomolecules wasanalyzed in a single experiment. A mixture of denatured Fetuin and RNaseB (denatured by boiling for 10 minutes in the presence of 1× RapidPNGase F buffer (New England Biolabs, Ipswich, Mass.) were pulled downby the immobilized Fbs beads conjugated with equal amount of WT, GYR orPPRYR Fbs proteins (FIGS. 10A and 10B). The bound fetuin and RNase Bwere eluted by 1× gel loading buffer, and analyzed by -PAGE/Coomassieblue staining. The amount of fetuin or RNase B on -PAGE gels werequantified using Image J. The ratio of Fetuin to RNase B (Fetuin/RNaseB) is arbitrarily defined as 1 in the input (FIG. 10A, Lane 7 and 10C).In low salt volatile buffer conditions, wild-type Fbs pulled down a verysmall amount of Fetuin (FIG. 10A, Lane1), and the Fetuin/RNase B is only0.29 (FIG. 10C), which indicates biased N-glycan linked glycomoleculebinding. In contrast, in the same low salt volatile buffer conditions,the GYR and PPRYR mutants pulled down significantly more Fetuin (FIG.10A, Lane 2 and 3) and the Fetuin/RNase B ration is very near to 1 (FIG.10C), indicating substantially unbiased in N-glycan linked glycomoleculebinding. In high salt volatile buffer wild-type Fbs pulled down moreFetuin (FIG. 10A, Lane 4) and the Fetuin/RNase B ratio is increased to0.92 (FIG. 10C), which indicates high salt conditions help to reduce thebias in N-glycan linked glycomolecule binding to wild-type Fbs. This isconsistent with the data shown in FIGS. 6A-C and 7A-C. For the GYR andPPRYR mutants, the high salt conditions are not necessary, since Fetuinand RNase B pulldown was not enhanced (FIG. 10A, Lane 5 and 6, and FIG.10C). The asterisk (*) indicates some SNAP-Fbs leached from theprototype immobilized Fbs beads.

Example 17. GYR and PPRYR Show Enhanced Binding to a Glycopeptide with aComplex N-Glycan (SGP-TMR) (FIG. 11)

SGP-TMR affinity to the Fbs in low salt volatile buffer was analyzed bya pull-down assay using SNAP Capture Pull-Down resin conjugated withequal amounts of WT, GYR or PPRYR Fbs proteins (FIG. 10B). The datashown as a bar graph in FIG. 11 was generated using the sameexperimental procedure as described in example 8. As compared to WT Fbs,GYR and PPRYR mutants show 3-fold and 2-fold increased affinity toSGP-TMR, respectively.

Example 18. Enrichment of N-Linked Glycomolecules from a Solution byApplication of Fbs Immobilized to a Solid Support, which is Compatiblewith Microfluidic Sample Processing

In the previous examples, the utility of Fbs has been described in thecontext of a SNAP-Fbs fusion protein, which is designed forimmobilization to benzyl guanine beads (SNAP Capture Pull-down resin).Fbs may also be employed alone or as a fusion to other proteinimmobilization modules. Fbs, an Fbs fusion protein or Fbs conjugated toanother type of immobilization module may be immobilized to theappropriate solid support. For example, Fbs or an Fbs fusion protein maybe immobilized within a channel of a microfluidic device designed forprocessing small sample volumes (picoliter to milliliter scale). In amicrofluidic device, a liquid sample containing N-linked glycomoleculeswould be allowed to come into contact with immobilized Fbs. The N-linkedglycomolecules of interest will bind and other molecules in the samplewould be washed away. After a washing step(s), 50% acetonitrile or othermass-spec friendly reagent may be added to elute the N-linkedglycomolecules of interest. A primary advantage of such a microfluidicdevice is the potential for automated sample processing in-line withautomated analysis of the N-linked glycomolecules of interest.

Example 19. Enrichment of N-Linked Glycomolecules from a Solution byMixing with Non-Immobilized Fbs

Fbs or Fbs fusion protein may be expressed as a recombinant protein thatis readily soluble in an aqueous solution and readily available forinteraction with a sample containing N-linked glycomolecules insolution. Separation of the N-linked glycomolecules of interest bound toFbs may be separated from the other molecules in the sample by severalmethods including filtration or binding of the complex of interest to asolid support via an immobilization module. In the case of filtration,the N-linked glycomolecules bound to Fbs may be captured on a filtermembrane with a molecular weight cut-off of 20 kDa or larger. Forexample, the Fbs SBD is greater than 20 kDa and the SNAP-Fbs fusionprotein is approximately 45 kDa. Therefore, N-linked glycomoleculesbound to the Fbs SBD or to an Fbs fusion protein will not pass throughthe membrane and other molecules less than 20 kDa will pass through amembrane with a molecular weight cut-off of 20 kDa or larger. After awashing step(s), 50% acetonitrile or other mass spec friendly reagentmay be added to elute the N-linked glycomolecules from Fbs. Thisapproach is envisaged for samples that have been processed by proteaseto create N-linked glycopeptides and/or processed with PNGase F torelease N-linked glycans. These 2 molecules types may be enriched andthen be eluted through the membrane for isolation and analysis. Thishypothetical application of Fbs for enrichment of N-linkedglycomolecules is similar to the work of Deeb, et al. who used a methodnamed “N-glyco FASP”, whereby lectin-based enrichment was used toquantify N-linked glycoproteins in lymphoma cells (Deeb, et al., MolCell Proteomics, 1, 240-51 (2014)).

For all patents, applications, or other references cited herein, such asnon-patent literature and reference sequence information (such asdatabase or accession numbers) are incorporated by reference in itsentirety for all purposes as well as for the proposition that isrecited. Where any conflict exits between a document incorporated byreference and the present application, this application will control.

As will be recognized by the person having ordinary skill in the artfollowing the teachings of the specification, the foregoing aspects canbe claimed by Applicant in any combination or permutation. To the extentone or more elements and/or features is later discovered to be describedin one or more references known to the person having ordinary skill inthe art, they may be excluded from the claims by, inter alia, a negativeproviso or disclaimer of the one or more elements and/or features.Headings used in this application are for convenience only and do notaffect the interpretation of this application or claims.

What is claimed is:
 1. A protein comprising a non-naturally occurringamino acid sequence having at least 95% sequence identity to SEQ ID NO:2 and mutations at least at two positions selected from positions thatcorrespond to positions 42, 43, 44, 61, and 62 in SEQ ID NO:2.
 2. Theprotein according to claim 1, comprising an immobilization module toform a fusion protein.
 3. The protein according to claim 1, wherein theposition of at least one of the at least two mutations are selected frompositions corresponding to amino acid 43, 61, and
 62. 4. The proteinaccording to claim 1, wherein the mutations at least at two positionsare selected from a Proline (P) at a position corresponding to aminoacid 42, an Alanine (A), Glycine (G) or Proline (P) at a positioncorresponding to amino acid 43, a Serine (S) or Arginine (R) at aposition corresponding to amino acid 44, a Tyrosine (Y) at a positioncorresponding to amino acid 61 or an Arginine (R) at a positioncorresponding to amino acid
 62. 5. The protein according to claim 3,wherein the position of at least one of the at least two mutations areselected from positions corresponding to Alanine (A), Glycine (G) orProline (P) at a position corresponding to amino acid 43, a Tyrosine (Y)at a position corresponding to amino acid 61 and an Arginine (R) at aposition corresponding to amino acid
 62. 6. The fusion protein accordingto claim 2, wherein the immobilization module is a variant ofO⁶-alkylguanine-DNA alkyltransferase (AGT).
 7. The protein according toclaim 4, fused to an immobilization module.
 8. The protein according toclaim 7, wherein the immobilization module is a variant ofO⁶-alkylguanine-DNA alkyltransferase (AGT).